Y Combinator has funded over 4,000 companies since 2005, producing household names like Airbnb, Stripe, Dropbox, Coinbase, and Instacart. Behind every successful YC company is a carefully chosen stack of tools and infrastructure that enabled them to move fast, ship constantly, and scale when traction hit. Understanding what these companies use — and why — gives any startup a blueprint for making smart tooling decisions.
We analyzed public information from hundreds of YC companies' job postings, engineering blogs, tech stack disclosures, and founder interviews to identify the most common tools across the YC ecosystem. Here is what we found.
Cloud Infrastructure: AWS Dominates, but GCP Is Gaining
Historically, AWS has been the default choice for YC startups, largely because of the generous AWS Activate credits available through the YC deal. Roughly 60% of YC companies use AWS as their primary cloud provider.
However, Google Cloud has been steadily gaining ground, now powering an estimated 25% of YC companies as their primary provider. The Google Cloud for Startups program — offering up to $200,000 in credits — has made GCP increasingly attractive, especially for startups building AI/ML products (Vertex AI, BigQuery) or using Kubernetes heavily.
Microsoft Azure accounts for roughly 10% of primary cloud usage among YC companies, with the remaining 5% split between providers like DigitalOcean, Render, and Railway for simpler workloads.
The trend is clear: while AWS remains the safe default, more YC startups are choosing GCP for its AI infrastructure and competitive pricing, especially those in the latest batches.
Frontend Frameworks: React and Next.js Lead
React is used by the vast majority of YC companies building web products — our estimate is north of 80%. Within the React ecosystem, Next.js has become the dominant framework, used by an estimated 50–60% of YC companies building new products since 2022.
The reasons are practical: Next.js provides server-side rendering (crucial for SEO and performance), API routes, file-based routing, and excellent deployment on Vercel. For startups that need to ship fast and rank well in search engines, Next.js eliminates a significant amount of infrastructure boilerplate.
Other frameworks making inroads include Svelte/SvelteKit (favored by developer-tool startups for its performance) and Remix (popular among teams that want more control over data loading patterns). But Next.js remains the overwhelming default.
Backend & APIs: Node.js, Python, and Go
On the backend, the YC ecosystem is split across three primary languages:
- Python: The most common backend language, used by roughly 45% of YC companies. Python dominates in AI/ML startups, data-heavy applications, and rapid prototyping. Frameworks like FastAPI and Django are the most popular choices.
- TypeScript/Node.js: Used by roughly 35% of YC companies, especially those with full-stack TypeScript codebases. Express, Fastify, and tRPC are common choices. The ability to share types between frontend and backend is a major productivity advantage.
- Go: Used by roughly 15% of YC companies, particularly those building infrastructure, developer tools, or performance-critical systems. Go's simplicity, strong concurrency model, and fast compile times make it popular for microservices.
Databases: PostgreSQL Is King
PostgreSQL is far and away the most popular database choice among YC companies, used by an estimated 70%+ as their primary relational database. The ecosystem around Postgres — including extensions like pgvector for AI embeddings, PostGIS for geospatial data, and managed services like Supabase, Neon, and RDS — makes it extraordinarily versatile.
For document storage, MongoDB remains popular (roughly 15% of YC companies), though its usage has declined in recent years as Postgres has absorbed many of its use cases through JSONB columns and flexible schemas.
Redis is the near-universal choice for caching and real-time features, used alongside the primary database by a majority of YC companies.
For startups building AI products, vector databases like Pinecone, Weaviate, and pgvector are increasingly common — roughly 20% of recent YC batches include companies using vector storage for embeddings and similarity search.
Authentication: Clerk and Auth0 Lead
Authentication is a solved problem that no startup should build from scratch. The most popular auth solutions among YC companies are:
- Clerk: The fastest-growing auth provider in the YC ecosystem, popular for its developer experience, pre-built UI components, and generous free tier. Many recent YC companies choose Clerk for new projects.
- Auth0 (Okta): The established choice, particularly for B2B SaaS companies that need enterprise features like SSO and SCIM.
- Supabase Auth: Increasingly popular among companies already using Supabase as their backend, since auth is built into the platform.
- NextAuth.js (Auth.js): The open-source option, popular among teams that want full control over their auth flow without vendor lock-in.
Payments: Stripe Is Universal
Stripe is used by virtually every YC company that charges money — our estimate is 90%+. Stripe's developer experience, documentation, and comprehensive feature set (payments, subscriptions, invoicing, Connect for marketplaces) make it the default choice. YC's deal with Stripe (waived fees on the first $100K of processing) further cements this position.
For companies operating globally or in specific verticals, Paddle and Lemon Squeezy have gained some traction as merchant-of-record alternatives that handle tax compliance automatically.
Analytics & Monitoring
The analytics stack for YC companies typically includes:
- PostHog: Rapidly becoming the default for product analytics among YC startups. Its open-source model, generous free tier, and all-in-one approach (analytics, session recording, feature flags, A/B testing) have made it a YC favorite.
- Amplitude: Still popular among growth-stage YC companies, especially for advanced behavioral analytics and cohort analysis.
- Sentry: The near-universal error monitoring choice. Sentry's startup program and free tier make it accessible from day one.
- Datadog: The most common full-stack observability platform for YC companies that have moved past the free-tier stage and need comprehensive monitoring.
Communication & Collaboration
Internal tooling at YC companies follows a remarkably consistent pattern:
- Slack: Universal for team communication (95%+ adoption)
- Notion: The dominant workspace for documentation, product specs, and knowledge management (70%+ adoption)
- Linear: The preferred issue tracker and project management tool for engineering teams (50%+ adoption, growing fast)
- Figma: Universal for design collaboration (90%+ adoption among companies with design teams)
- Loom: Widely used for async video communication, product demos, and internal documentation
AI & ML Tools
Given YC's heavy investment in AI startups (roughly 60% of recent batches involve AI), the AI tooling stack is particularly relevant:
- OpenAI API: The most commonly used LLM provider, with startup credits available through various programs
- Anthropic Claude: Rapidly gaining adoption for its strong reasoning capabilities and startup credit programs
- Hugging Face: The default platform for open-source models, hosting, and inference
- LangChain / LlamaIndex: Popular frameworks for building AI applications with retrieval-augmented generation (RAG)
- Replicate: Used for running open-source models like Stable Diffusion, Llama, and Whisper via API
Building Your Own YC-Grade Stack
You do not need to be a YC company to use the same tools. Most of the tools listed above offer generous free tiers or startup credit programs that any founder can access. The key takeaway from the YC ecosystem is not which specific tools to use, but the principles behind the choices:
- Choose tools that let you move fast. Developer experience matters more than feature completeness at the early stage.
- Use managed services over self-hosted. Your engineering time is too valuable to spend on ops when there are free or cheap managed alternatives.
- Default to the ecosystem standard. Using popular tools means better documentation, easier hiring, and more community support.
- Claim every credit program you qualify for. Visit our complete credits directory to find programs from every tool mentioned in this article.